The term pig is used to refer to devices that are passed through pipelines or tubing whether for cleaning the pipelines or for monitoring the internal surfaces and thickness of the pipes or tubes and for separation of product within the pipe or tube. This invention is particularly concerned with pigs that can be used to clean the internal surfaces of pipelines or tubes from the inside usually by scraping debris from the internal surface.
Although the invention is particularly useful with tubing used in oil refinery furnaces for carrying the hydrocarbons that are to be subject to high temperatures, it may also be used in connection with other pipes and tubing.
The tubing systems in refinery furnaces such as those used for crude oil distillation, vacuum thermal crackers, visbreakers, delayed cokers and the like typically have a sinusoidal path through the furnace to optimise the exposure of the contents of the tube to the heat; this is frequently referred to as the furnace coil being serpentine. In a typical furnace or process fired heater the product to be treated usually passes downwardly through the tube system and in some furnaces the initial section of the tubing consists of an upper closely packed tubing section in which the temperature of the product to be treated is raised to the treatment temperature by convection heating. Typically the pre-heated product then passes down to a lower section of the tubing in which there is more space between the lengths of tubing and in this section the tubes are heated by radiant heat. Typically, in both sections the tube or process fired heater consists of straight sections joined by semicircular bend sections, known as u-bends.
In order for efficient and safe operation of such a tubular system it is important that the tubes are periodically cleaned to ensure that the walls of the tube are free from undesirable deposits which will likely lead to inefficient heat transfer to the contained fluid.
Traditionally furnace process tubes have been cleaned/decoked using the method known as ‘steam air decoking’. More recently, since the mid 1990's, mechanical decoking or pig decoking has gained in favour in oil refineries around the world, widely replacing the practice of ‘steam air decoking’. Mechanical decoking is carried out by driving an abrasive or scraper pig through the pipe or tube to scrape deposits from the internal surface of the pipe or tube. This can be accomplished by driving the abrasive pig through the tube under fluid pressure such as water pressure. For example pumping unit machinery having water tanks and pumps can be delivered to a refinery, linked up with the tubing within a refinery furnace to produce a circuit through which the cleaning pig may be driven under water pressure so that the debris obtained by the cleaning operation is removed from the tubing system in the water stream and can be separated from the water for disposal. The cleaning operation may be performed by several runs of the cleaning pig which can be in the same direction or in opposite directions. After the cleaning operation the tubing system may be inspected and traditionally this has been performed in a separate operation.
As previously mentioned it is known to send a pig through a pipeline for the purpose of clearing any blockage therein and for removing unwanted deposits that have formed on the inner wall thereof. Such a device finds application, for example, in the oil industry, especially for cleaning fired heater or furnace tubes in a refinery. Refinery fired heaters may be subjected to temperatures normally in excess of 200° C., and in specialist furnaces temperatures can exceed 700° C. Such conditions lead to the formation of carbonaceous deposits (coke) on the pipeline wall. A pig can then be forced therethrough under pressure of a fluid, for example water, such that the deposits are removed by friction as the pig scrapes along the pipeline wall. U.S. Pat. No. 5,924,158 discloses an exemplary pig suitable for this purpose. The pig may be passed through the pipeline, uni-directionally or bi-directionally, several times to remove the coke. This type of decoking is carried out after the furnace has been taken out of service and cooled down.
Examples of scraper pigs are given in U.S. Pat. No. 5,924,158; U.S. Pat. No. 5,150,493 and U.S. Pat. No. 4,244,073. According to U.S. Pat. Nos. 5,924,158 and 5,150,493 a ‘scraper pig’ may be driven through a furnace coil or pass by a controlled flow of a fluid, typically water. Both inventions are designed to scrape away hard deposits of coke and other material; both have appendages extending from the body of the pig for scraping the contamination away from the inner surface of the tube or piping and both are designed to be flexible to be able to negotiate short radius 180° U bends and box plug header direction changes. Typically the appendages are scraper studs mounted in the outside of the pig to scrape the internal surface of the tubing with which they are used.
In U.S. Pat. No. 5,150,443 a captured thread for screwing in various types of scraping appendage is provided. U.S. Pat. No. 5,924,158 provides a system of anchoring studs which uses compressed air to inflate the plastic casing in order to provide a secure and tight housing for the ‘plurality of studs’. Both these systems have limitations in their design.